Permselective membranes for gas separation are known and used commercially in applications such as the production of oxygen-enriched air, production of nitrogen-enriched-air for inerting and blanketing, separation of carbon dioxide from methane or nitrogen for the upgrading of natural gas streams, and the separation of hydrogen from various petrochemical and oil refining streams. The separation of gases by polymeric membranes is thought to depend on the size of the gas molecules and the physical or chemical interaction of the gas with the material of the membrane. For certain gas streams, one or more component or minor contaminant may exhibit a strong interaction with the material of the membrane, which can plasticize the membrane. This can result in reduced production rate and selectivity, and ultimately, loss of membrane performance. A membrane with a good balance of high production rate and selectivity for the gases of interest, and persistently good separation performance despite long-term contact with aggressive stream composition, pressure and temperature conditions is highly desired.
U.S. Pat. No. 4,705,540 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and a rigid dianhydride or mixtures thereof, specifically pyromellitic dianhydride (PMDA) and 4,4′-(hexafluoroisopropylidene)-bis(phthalic anhydride) (6FDA). These polyimides form membranes with high gas permeabilities but fairly low permselectivities. These polyimides are also sensitive to various organic solvents.
U.S. Pat. No. 4,717,393 shows that polyimides incorporating at least in part 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and phenylene diamines having substituents on all positions ortho to the amine functions can be photochemically crosslinked. Membranes formed from such photochemically crosslinked polyimides have improved environmental stability and superior gas selectivity than uncrosslinked polyimide. However, photochemical crosslinking is not a practical method for fabricating gas separation membranes cost-effectively.
U.S. Pat. No. 4,880,442 discloses highly permeable polyimide gas separation membranes prepared from phenylene diamines having substituents on all positions ortho to the amine functions and essentially non-rigid dianhydrides. These polyimides again exhibit high gas permeabilities, but low permselectivities.
Bos et. al., AIChE Journal, 47,1088 (2001), report that polymer blends of Matrimid® 5218 polyimide (3,3′,4,4′-benzophenone tetracarboxylic dianhydride and diaminophenylindane) and copolyimide P84 [copolyimide of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride and 80% toluenediisocyanate/20% 4,4′-methylene-bis(phenylisocyanate)] can increase the stability of the membrane against carbon dioxide plasticization when compared to the plain Matrimid® 5218 membrane. They do not disclose any other polyimide blends used for gas separation however.
U.S. Pat. No. 5,055,116 describes a blend of aromatic polyimides, in which the proportion of the polymer components is adjusted to achieve certain permeability and selectivity of a polymer membrane. The final properties of a new polymer membrane may be predicted so that a membrane with those desired final properties could then be manufactured. U.S. Pat. No. 5,055,116 indicates that the gas transport properties of the membrane prepared from the polyimide blends are predictable and the membrane may be “engineered” to achieve the desired final properties. To the contrary, the gas transport properties of the present invention are unpredictable and surprisingly good.
U.S. Pat. No. 5,635,067 discloses a fluid separation membrane based on a blend of two distinct polyimides. One is the copolymer derived from the co-condensation of benzophenone 3,3′,4,4′-tetracarboxylic acid dianhydride (BTDA) and optionally pyromellitic dianhydride (PMDA) with a mixture of toluene diisocyanate and/or 4,4′-methylene-bis(phenylisocyanate). The other is Matrimid® 5218 polyimide.
The permeation properties of miscible polymer blends can be estimated from the following equation 1 (D. R. Paul and S. Newman, “Polymer Blends”, Vol. 1, Chapter 10, p. 460, Academic Press, New York, 1978, B. G. Ranby, J. Polymer Science, Part C 51 p. 89, 1975, A. E. Barnabeo, W. S. Creasy, L. M. Robeson, J. Polymer Science, 13, p. 1979, 1975):                               ln          ⁢                                          ⁢                      α            B                          ≈                              ∑            i                                                          ⁢                                          ⁢                                    φ              i                        ⁢            ln            ⁢                                                  ⁢                          α              i                                                          (        1        )                            where:                    αB is the blend permeability or selectivity,            φi is the volume fraction of component i, and            αi is the permeability or selectivity of each blend component.                        
For most blends cited by Paul and Newman, measured permeation performance corresponded reasonably well with permeation performance calculated by Equation 1. Therefore significant deviations of actual performance either over or under calculated performance predicted by Equation 1 indicates unusual behavior of the blend.
It is desirable to have polymeric gas separation membranes that exhibit high gas permeation rates while maintaining high relative gas selectivity. However, prior art membrane materials generally compromise one for the other. A major challenge for researchers in this field has been to develop materials that show either an increase in permeability with little sacrifice in selectivity, or an increase in selectivity with little sacrifice in permeability.